Effect of Bullet Spin on Terminal Performance

Discussion in 'Long Range Hunting & Shooting' started by Tiny Tim, Aug 17, 2019.


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  1. jasent

    jasent Well-Known Member

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    Nov 16, 2010
    From what I’ve observed faster twist does have better terminal performance. I have always gone faster than “needed” when given the choice. Jme
     
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  2. GeorgeS

    GeorgeS Well-Known Member

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    Nov 26, 2018
    Update to earlier post: I mentioned that the internal forces on a bullet spinning at a high rate are working to tear the bullet apart - which, in the old days, with .220 Swift, happened. The bullets would disappear in a puff of lead smoke downrange, until they were built to take the forces trying to spin them apart.

    I started working up a spreadsheet using Excel, my creaky memory of physics, and my fading concentration - and then it occurred to me that there's this thing called the Internet (D'oh!). I found an on-line centrifugal force calculator, and plugged in some figures for a Swift. Turns out the centrifugal acceleration is about 160,000 G's. The force - since the bullet is small and light, comes to about 1,250 lbs-ft. That's a 55 grain at 4,400 f/s in a 1/14" barrel.

    For reference, 1,250 lbs-ft is the equivalent of the torque developed by the 5 liter V8 in a 2019 Ford F-150 - times 3 - although how you translate between the two beats me. It's pretty clear that if that on-line calculator is right, the materials toughness of that bullet, resisting the force trying to tear it apart in flight, is steroidal.

    Just thought I'd throw this into the stew pot.
     
  3. crazyhorse

    crazyhorse Well-Known Member

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    Oct 31, 2004
    If energy transfer is a primary component of success then rotational speed (any speed for that matter) is definitely an important and contributing component. Aside from the bullet itself disrupting/moving tissue to create a pathway for itself, the tissue that is being moved has the same effect on the tissue its replacing and so on. This same component applies on an expanding bullet (albeit on a smaller scale) to the tissue being moved by the open/opening petals or fragmented pieces moving away from the projectile due to disruption and rotation are having the same effect.
    Watching a slow motion video shows how the rotating striations in the gel block represent the transfer of rotational energy in addition to the impact of the projectile itself, and the faster it is rotating the more energy transferred.

    Rotation is also a vital component of the little bullet mushrooms we have grown accustomed to seeing. Due to the extremely fast rotation of the bullet, when it hits something causing physical disruption the "petals" are moved outward due to the rotational energy being imparted as it moves through the medium.
    Take a bullet like the Hammer hunter, if it were possible to shoot it into a gel block knuckle ball style it would likely just start to pile up on itself as it passes through the gel. The hollow point filling with gel would have some initial effect of moving the material outward but without the extreme rotation, not nearly enough to completely overcome the taper of the bullet required to achieve the desired BC.
     
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  4. dennisinaz

    dennisinaz Well-Known Member

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    Jan 18, 2012
    No, Bergers were designed from the start to be very accurate bullets, the popular VLD bullets with the ORIGINAL jacket thickness (Orange box now) were just long range target bullets. Berger came up with an idea to MAKE a hunting bullet and even announced it at SHOT. I was given some sample bullets to test on game etc. The Bullets were called Bergerlock or something like that. They never became a reality. Berger was headquartered in Phx and it was nice to pick stuff up at the factory- reduce rough handling, etc.

    Hunters started using the VLDs on game because they were good in the wind and were accurate. They , the hunters, discovered that the long meplat was delaying the explosion long enough that they killed magnificently. They were NEVER designed as a hunting bullet. They later came out with the heavier jacket bullets because people were pushing them too hard and tearing up the thin jackets.
    Berger moved to Cali and really grew in size. Eventually they were bought out by the same company that owns Lapua and manufacturing moved back to Phx (Mesa, actually).

    I have killed and seen killed several hundred head of game killed with various Berger VLD and Hybrids. They work really well MOST of the time. My buddy shot a pronghorn this weekend with a 180 out of a 7RUM. It zipped right through but destroyed a lot of stuff on the way. I have seen big bears, big bull elk and LOTS of little coues deer killed with the 180s.

    I know this isn't a thread about Bergers but they perform best if they are twisted like Berger recommends. A lot of times I was able to shoot them in rifles that were not twisted fast enough but at higher elevations in thin air they would stabilize and shoot amazingly well. They almost always were pass-throughs and not nearly as lethal as a properly spun up bullet.

    Do some media testing with a reduced load to simulate a long shot and them do the same test at range. You will be shocked at the difference in performance. Shortcuts do not always tell the truth. The denser the medium the bullet is traveling through, the more spin stability is required. Thin, high desert air is very forgiving to marginally stabilized bullets. Thick, cold, coastal air is a real test of the stability.

    Now just imagine how much denser an animal is than the air. Spin helps it go straight and helps it upset. Too much spin is not perfect as it will also play heck with accuracy sometimes. Look at the minimal amount of spin the short range benchrest guys use- 30 calibers with 15, 16 and 17" twists! They know the best accuracy is right on the ragged edge of stability but the TERMINAL performance is just the opposite!